The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) has developed a set of new standards covering wavelengths and signal formats in order to better support the multiplexing of a substantial number of signals onto a single fiber. These signal format and hierarchy standards cover digital signals and include the Operations, Administration, Maintenance, and Provisioning (OAM&P) overhead as part of the signal format. In the context of this disclosure, Optical Transport Network (OTN) refers to networks using the ITU-T Recommendation G. 709/Y1331, Interfaces for the Optical Transport Network (03/2003) Recommendation G. 709 standard for Wavelength Division Multiplexed (WDM) signals.
WDM transport networks based on the ITU-T OTN standards are becoming increasingly important. The reasons carriers are moving toward OTN include: OTN is a much less complex technology for transport applications than synchronous optical networking and synchronous digital hierarchy (SONET/SDH). The OTN signal incorporates overhead optimized for transporting signals over carrier WDM networks. The combination of the reduced technology complexity and optimized overhead allows substantial reductions in carrier transport network operations expenses. The OTN multiplexing bandwidth granularity is one or two orders of magnitude higher than for SONET/SDH, thus making it more scalable to higher rates. OTN now provides a cost effective method for carrying high-speed wide area network (WAN) data clients including Ethernet and storage area network (SAN) protocols. OTN provides an integrated mechanism for forward error correction (FEC) that allows greater reach between optical nodes and/or higher bit rates on the same fiber. Client signals can be carried over OTN in a transparent manner. This transparency includes native SONET/SDH signals for the “carrier's carrier” application where the entire client SONET/SDH signal's overhead is preserved through the OTN.
In other words, as illustrated in FIG. 1, OTN provides an optimum converged transport technology for transparently carrying important legacy and emerging client signals.
Network elements (NE) within a telecommunications network will need to switch and aggregate Constant Bit Rate (CBR) clients such as SONET/SDH, STS-N/STM-M, OTN optical channel transport unit (OTUk), and video streams and data clients such as Ethernet and Fibre channel. Historically, CBR clients are switched using an asynchronous crossbar, while Data clients are switched by a packet switch. Over the lifetime of a network element (NE, also called a network node), the relative mix of CBR and Data traffic will change. The NE may start out as serving mostly CBR clients and evolve to serving mostly data clients, or vice versa. In order to seamlessly support the above evolutions, a NE with separate CBR and Packet fabrics would have to install 100% bandwidth in both fabrics. It is more cost effective to use a single combined switch fabric that can handle both CBR and Data traffic. Due to the higher growth rate of Data traffic, the current fabric of choice in an NE is a packet fabric.
A packet fabric network element can switch a CBR client by receiving a CBR client at an Ingress card, which converts the CBR client data stream into packets, which are routed to an egress card via the packet fabric. The egress card then reassembles the CBR client from the packets and transmits the CBR client to a downstream NE. However, the process of converting the CBR client data stream into packets loses the phase and frequency information of the CBR client, which is a problem as the outgoing CBR client must match the phase and frequency of the incoming CBR client.
In the application of switching a CBR client through a network element (see FIG. 2, to be described later in further detail), there are a number of known ways to control the transmit clock to be phase-locked to the CBR client.
However, there are problems or disadvantages associated with prior art techniques that have been developed for signaling this phase and frequency information from the Ingress card to the Egress card, so that the Egress card can correctly recreate the CBR from the packets.
One of these prior art methods is to construct packets with fixed amount of CBR client data, transfer the packet through the fabric, and write the CBR client data into a first in first out block (FIFO) and read from the FIFO at the transmit clock rate. The transmit clock is sped up or slowed down depending on whether the FIFO depth is above or below an equilibrium threshold. A highly related scheme is to use a phase discriminator to monitor packet arrivals and compare it against the transmit clock. The phase discriminator will get a phase dump equal to the amount of data in each packet as packets arrive. Both of the above schemes are subject to the effects of packet delay variation (PDV). In particular, if the delay through the fabric changes, the transmit clock will experience a phase hit which is seen by the CBR client as wander. These methods are commonly known as Adaptive Timing.
Another known method of signaling phase over a packet fabric is to timestamp each packet. For example, the creation time of each packet can be logged and inserted into overhead bytes of the packet. This scheme has the benefit of being insensitive to PDV. However, because each packet carries data and control information (the timestamp), the egress port must segregate the two components when it re-generates the CBR client stream. In addition, the potential throughput of the fabric is reduced by the amount of bandwidth consumed by the timestamp. Another key requirement for this methodology is that the source and destination of the data must have access to the same reference clock. This common clock controls the rate at which the timestamp counter increments. This method is commonly known as Differential Timing.
In the application of transporting a CBR client through a SONET/SDH or OTN network, there are a number of known ways to control the transmit clock at the egress NE to be phase-locked to the CBR client received by the ingress NE.
One method is to construct General Framing Protocol (GFP) frames with fixed amount of CBR client data, transport the GFP frames through the network, and write the CBR client data into a FIFO and read from the FIFO at the transmit clock rate. The transmit clock is sped up or slowed down depending on whether the FIFO depth is above or below an equilibrium threshold. A highly related scheme is to use a phase discriminator to monitor packet arrivals and compare it against the transmit clock. The phase discriminator will get a phase dump equal to the amount of data in each packet as packets arrive. Both of the above schemes are subject to the effects of how GFP frames of multiple CBR clients are multiplexed onto a single SONET or OTN path. Consider the case where three CBR clients (Clients A, B and C) are multiplexed using the GFP Extension Header facility defined in ITU G.7041. If the multiplexing order had been A, B, C over an extended period of time and then switches to C, B, A the arrival times of GFP frames of Client C would suddenly be earlier than expected and arrival times of Client A would be later. The transmit clock of Clients A and C will experience a phase hit which is seen by the CBR client as wander.
Another known method of signaling phase over a telecommunications network is to timestamp each GFP frame in a manner analogous to SRTS in asynchronous transfer mode (ATM) networks (U.S. Pat. No. 5,260,978, which is incorporated by reference in its entirety). For example, the creation time of each GFP frame can be logged and inserted into overhead bytes of the frame. This scheme has the benefit of being insensitive to PDV. However, because each packet carries data and control information (the timestamp) the egress port must segregate the two components and process them separately. It would not be possible for an NE using timestamps to inter-operate with one that does not.
It is, therefore, desirable to provide methods and apparatus which allow for the better transmission of phase information relating to packetized CBR data streams.